In a process of manufacturing an automobile, a shaft through-hole through which a crankshaft is inserted is cut out in a cylinder block of an engine. Subsequently, a cylinder head and a crank case and the like are assembled to the cylinder block.
The shaft through-hole is formed in the following procedure.
First, a cutting tool having a cylindrical tool body and a plurality of cutting blades on an outer circumferential surface of the tool body is prepared. The cutting tool includes a main flow path, which extends in a longitudinal direction, and a first branch flow path which diverges from the main flow path and reaches a vicinity of each cutting blade provided on the outer circumferential surface.
Then, a plurality of through-holes serving as pilot holes spaced from one another are formed in the cylinder block. Subsequently, the cutting tool having the cutting blades is inserted in the pilot holes. In this state, the cutting tools are rotationally driven to simultaneously cut inner circumferential surfaces of the plurality of pilot holes, that is, to perform what is called “line boring”. Consequently, shaft through-holes are concentrically formed on the pilot holes.
At that time, heat and chips are generated at a machined part of the inner circumferential surface of the through-holes by the cutting blades. The heat and the chips are eliminated by spraying machining liquid from a vicinity of each cutting blade.
In such line boring, the cutting tool is rotationally driven by holding one end thereof in a cantilever state. Thus, there have been problems in that the cutting tool is downwardly curved under its own weight, that the machining precision of the shaft through-hole is degraded, and that the cutting tool vibrates.
Accordingly, a technique for supporting the other end of the cutting tool by a support jig has been proposed (see Parent Document 1).
In the apparatus disclosed in Patent Document 1, a second branch flow path formed so as to reach a part of the outer circumferential surface from the main flow path, which faces a bushing face of a support jig, is provided in the cutting tool. Then, machining liquid is sprayed onto the bushing face of the support jig through the second branch flow path. Thus, a hydrodynamic bearing is formed between the bushing face of a support jig and the outer circumference surface of the cutting tool.
However, according to the apparatus disclosed in Patent Document 1, sometimes, an inside diameter of the second branch flow path cannot be set at an appropriate value. Thus, the support stiffness of the hydrodynamic bearing is low.